Cinnamon and Brewer's yeast extracts have been shown to potentiate the action of insulin in isolated adipocytes. In this study, isolated rat epididymal adipocytes were used to evaluate the influence of bovine serum albumin on insulin activity as affected by cinnamon and Brewer's yeast extracts. Albumin at 0.01-0.1% decreased the insulin stimulatory effects of cinnamon from 11.8- to 5.3-fold and 2% albumin decreased this effect to near control levels. Conversely, the insulin-enhancing properties of Brewer's yeast remained low in the presence of less than 0.25% albumin but subsequently increased 2.8-, 4.8- and 5.6-fold in the presence of 0.25, 0.50 and 1.0% albumin, respectively. In the absence of added insulin, increased activity of the insulin-stimulated utilization of glucose by both extracts was observed but only Brewer's yeast extract displayed additive effects when tested at higher insulin levels. Due to the inhibitory and enhancing effects of albumin on the insulin activity of cinnamon and Brewer's yeast, respectively, it is suggested that the effects of albumin be assessed when evaluating the insulin-enhancing effects of other substances using isolated adipocytes.